In the treatment of municipal wastewater, a sludge is produced which contains a substantial amount of degradable organic matter and nutrients. Beneficial reuse of this sludge cake, or biosolids material is practiced world-wide.
Wastewater enters a wastewater treatment plant as influent. This influent is typically subjected to some form of biological nutrient removal, after which the solids (sludge) are separated from the liquid (effluent) portion by some type of clarification, or settling process. The effluent can be filtered, disinfected, and discharged to surface waters or reuse. The sludge portion is traditionally transferred to some type of stabilization process, where the degradable organic matter is broken down to a non-putrescible biosolids material for disposal. Three of the most common processes used for stabilization of municipal sludge in the U.S. are anaerobic digestion, aerobic digestion, and composting.
In the process of anaerobic digestion, microorganisms break down the biodegradable material in the sludge in the absence of air. The end products of this biological degradation are a stabilized biosolids, and biogas, which is largely comprise of methane and carbon dioxide. The quantity of methane resulting from the process is directly related to the biological oxygen demand or chemical oxygen demand of the sludge consumed in the process.
While the carbohydrate and lipid component of sludge is easily degraded, the proteins are contained inside the cell walls, which must first be broken down to make the contents readily available as a nutrient for digestion. Attempts have been made related to the pre-treatment of the sludge in order to facilitate the process of anaerobic digestion. One example is the Cambi® process (U.S. Pat. Nos. 5,888,307 and 6,913,700), which uses thermal hydrolysis (high pressure steam) to make nutrients more readily available. Earlier examples include the Portous and Zimpro processes, which date back to the 1940's-60's. Thermal hydrolysis typically involves pre-treatment of the waste activated sludge by elevating temperatures of the sludge to ˜135° C. for 1-2 weeks, or higher temperatures (185-200° C.) for 30 minutes. Another example of a sludge pretreatment process is the OpenCel® process, which uses pulsed, high-voltage (20-30 kV) to disrupt cell membranes and cell walls. Yet another example is the Crown® Disintegrater, which utilizes mechanical forces to shear cells and disrupt cell walls and membranes. Each of these processes are similar in that they all require high capital and operating costs.
The inventors have realized that it would be desirable to have an improved method of hydrolyzing or solubilizing sludge which results in high conversion rates of degradable organic material to biogas, has low capital and operating costs and is scaleable.
The present invention represents an innovative process which offers superior performance for the intended purpose and has extremely low capital and operating costs. This results in an obvious economic advantage over previously known methods.